Analytical system



. or vapor form.

' ion collector.

Patented May 21, 1946 UNITED sTA'rfi-:SCPATENT OFFICE ANALYTICAL SYSTEMReed C. Lawlor, Alhambra, Calif., assignor to Con. solidated EngineeringCorporation, Pasadena, Calif., a corporation of California ApplicationJuly 31, 1942, serial No. 453,128 .3 ,claimsl (Cl. 11s-1s) My inventionrelates to mass spectrometry, and more particularly to the analysis of achemical mixture containing a plurality of components which uponionization may form ions having the same mass-to-charge ratio.More-specically, my method makes possible the analysis of such a mixtureby taking into account the fact that such ions are usually produced indifferent pro- .portionsv from such components ionization voltages.

This application is acontinuation-in-part of copending patentapplication, Serial No. 378,636, iiled February 12, 1941.'

When analyzing a mixture (which originally may be gaseous. liquid orsolid) with a mass at different spectrometer, the mixture is introducedfrom a.

sample region into an ionization region in gaseous 'In the ionizationregion, molecules of the mixture are ionized in characteristic'waysunder controlled conditions by subjecting them to the ionizing action ofparticles such as electrons. Such electrons are commonly directed by theaction of `an electric field into the ionization region where theyencounter and ionize molecules -of the mixture. The ions formed are thenwithdrawn into an analysis region where ions of different mass-to-chargeratios are 'segregated into beams which may be detected bysuccessive'focusing thereof upon an As the 'beams are successivelydetected at the collector, the'beam intensities may either be measureddirectly with a suitable indicator or else automatically and permanentlyrecorded prior-to measurement. The peak -intensities of the ion beamsare representative of either the amounts of the withdrawn ions or theratesl of formation of the respective ions. Such a record or set oimeasured peak intensities forms a mass spectrum.

For simplicity, the ionization of a sample by particles having a givenamount .of energy corresponding to the kinetic energy of electrons whichhave been accelerated by somepredetermined voltage will be referred tosimply .as ionization of the sample or component at that voltage.-

Either before'or atterra mass spectrum of a mixture has been "obtained,reference samples containing different proportions of the componentsthat may be present in the mixture or substances correspondingchemically thereto are similarly subjected to analysis in a massspectrometer under substantially the same ionizing conditions to whichthe mixture was subjected, and corresponding mass spectra obtained forthe reference samples. Usually, though not necessarily, the referencesamples are relatively pure samples of the respective components thatmay occur-.in the mixture.

The composition of the mixture is then determined by comparing the massspectrum of the mixture with the mass spectra oi the reference samples.Such a comparison is most simply carried out by a mathematical procedureinvolving the comparison of the peak intensities o! the beams formingthe spectra.

' In practice, the analysis of such a mixture is greatly facilitated ifthe intensity of each peak occurring in the mixture spectrum representsthe sum of the intensities of corresponding peaks that would be obtainedin mass spectra of the separate components ii present alone. A methodfor producing this eiect, which is known as linear superposition, isdisclosed and claimed in copending patent application, Serial No,320,802, illed by Harold W. Washburn and Daniel Dwight Taylor. Briefly,this method involves maintaining the sample in a sample chamberhomogeneous at al1 times during analysis, owing the components from thesample chamber in to an ionization chamber through a gas inlet atmutually independent rates, ionizing each component in proportion to itspartial pressure in the ionization chamber and independently of theamounts of the other components there, and pro# viding such pressureconditions in the mass spectrometer that collisions between ionswithdrawn from the ionization chamber with any molecules, either in theionization chamber or in the analyzing chamber, are relativelyinfrequent.

When conditions suitable for achieving linear superposition during theanalysis of a mixture are obtained, the intensity of any peak occurringin the mass spectrum of the mixture due to ions of mass-to-charge ratiom produced at a voltage V may be represented by the following equation:

e X=the partial pressure of component i in the vso mixture and VP"=asensitivity coefficient representing the efficiency of the massspectrometer in producing ions of mass-to-charge ratio m obtained fromcomponent i by particles having an ionization energy of V electronvolts.

made by obtaining mass spectra of known mixtures approximating theunknown in composition and comparingA the spectrum of the unknownmixture vwith the spectra of the known -mixture by the interpolationmethod which is more fully explained in copending patent application,Serial No. 324,950, led MarchA 20, 1940, by Daniel Dwight Taylor.

For simplicity, I shall discuss the application of my inventionunderconditionsv such that linear superposition is maintained..

The amount of ions of mass-to-charge ratio m `originating from a givencomponent i is found to vary over a wide range with changes in theioniz- According to the present invention, I ionize the mixture atvoltages at which such ions arev formed in diierent proportions fromdifferent components, measure the amounts of ions of such mass--to-charge ratio formed at each voltage, and. utilize those measurementsin determining the com-` position of the mixture.

My invention may be applied, forexample, to the analysis of mixturescontaining hydrocarbon isomers, which generally produce ions of the samemass-to-charge ratios, and to mixtures con?v taining other components ofthe .same molecular weights.

Accordingly, the principal object of my invention is to provide animproved method for analyzing chemical mixtures with' a massspectrometer. .My invention possesses numerous other objects andfeatures of advantage, some fwhich together with the foregoing will beset forth in the following description embodying and utilizing my novelmethod. It is therefore to be understood that my invention is applicableto analyses of a variety of mixtures and to other mass spectrometers -ofvarious types or which may utilize other kinds of ionizing particles,and that'I do not limit myself, in any way, to the analyses, to theapparatus, or to the ionizing particles, of the present application, asI may adopt various other modications of my invention utilizing themethod, within the scope of the appended claims. My invention may bemore readily understood by direct reference to the drawings in which:

Fig. 1 shows a general organization of a mass spectrometer to which mymethod may 4be applied.

Fig. 2 is a schematic drawing partly showing a section taken on a line2-2 of Fig. 1 of part of the mass spectrometer including the ionizationchamber and the ionization energy controls.

Fig. 3 is a graph showing ionization curves of a mixture of twocomponents and of the components alone.

Referring to the drawings:

In Fig. 1, I have shown a sample'chamber I connected to an ionizationchamber 3 through a which a sample may be admitted; A pressure gauge,for example a McLeod 'gauge I 3, isconnected to the sample chamber, Avalve I5 is provided in the line 5 which connects the sample chamber tothe ionization chamber.

A s illustrated in Figs. 1 and 2, electrons emitted from a. heatedfilament I1 are directed in a beam I8 through aperture I9 in theelectron beam intensity control electrode 2|, through aperture 2 3 inelectronaccelerating electrode 25 which is formed by -part ofthe wall ofsaid ionization chamber 3, and through aperture 21, in the OPPO- siteportion of the wall of said ionization chamber, said beam being directedalong the line perpendicular to the face of a magnetic pole 29 by thecombined'action of jthe magnetic -eld indicated by arrow 3I in Fig. 2,which `field is directed downward perpendicular to the plane of thetrodes 2I and .25.

drawing in Fig.1. and electric fields parallel to the magnetic eld. Theelectrons passirigthrough said apertures impinge on electron-*catcher 32electrically connected to electrode 25.' The'electric iields areprovided by potentials applied from voltage supply circuit 33 tofilament I'I and elec- Said voltage supply circuit includes .means forvarying the relative potentials between said iilament and saidelectrodes.

One way of accomplishing this variation is tp connect the lament to thenegative ends of a. pair of parallel connected rheostats 35 and'31through whichcrrents ilow from a battery `v39,

and to connect said Aelectrodes-2| and 25 to electron beam intensity.'The amounts of ions produced are dependent on the energy of theelectrons in beam I8. t

The positive ions formed areaccelerated toward first slit electrode 4Ibyaction of a small electric potential which maintains said iirstcollimating tube 5 containing a gas inlet I which may be in the form ofa capillary leak or restricted orice. A gas sample to be analyzed isintroducedwhen the mass spectrometer is in condition for operation intothe ionization chamber through. said inlet. A T shaped conduit I0 isconnected to the sample chamber I. One branch of the T is con nected toa vacuum pump through a valve II.

slit electrode negative with respect to a pusher electrode 43 on theopposite sidetof said electron beam. Some'of the accelerated ions passthrough a narrow slit 45 in said iirst collimating elec'- trode 4 I andare thereupon acceleratedby a large negative potential between saidfirst collimating slit electrode 4I and second collimating slitelectrode 41. Some of the accelerated ions then pass through asecond-slit 49 in said second 'collimating slit electrode.

The ions thus pass through slit 49 at a high velocity. Thepath they'take,however, is not straight but curved, owing tov-the action of thejustnienaeither'of the magnetic neia-,orrofsthe accelerating voltagaorof both, ions- M-Tan'yle' sired massfto-char'ge ratio' can v'bemade'to'f'ollow a predetermined radius of curvature varid *thus befocusedat exit sli t 5 I positioned in* front-'ot an ion collector 53.AWhaha containing ions of' predetermined mass-to-chargeIA ratio Theother branch of the T has a valve 9 through 75 impinges on collector 53,a corresponding ion cur- 3,609,557 rent is produced, which current maybe measured by a suitable vacuum-tube amplier it and gal; vanometer-B'l,or recorder (not shown).

In order to obtain a mass spectrum of a sample, beams of differentmass-to-charge ratios are successively moved past said collector slit 5|by changing voltages supplied to said pusher electrode 43, and saidcollimating slit electrodes 4I and 41, by electrical connection tosuitable points of a potential divider t in ion beam deflection controlcircuit Si. This may be done most readily by changing the voltage acrosssaid divider Il by adjustment of a sliding potentiometer contact 6lconnected to one end thereof. When beams comprising' ions of differentcorresponding mass-to-charge ratios are successively focused at saidexit slit i. ions in the different beams are successively discharged atthe ion collector 53, the respective intensities. of corresponding ioncurrents are read on galvanom-v which is a, function of the ionizationvoltage.

From a knowledge of these cracking patterns and their variation withionization voltage, to-

gether with a knowledge of isotope ratios and packing fractions of theions recorded, the quantities ol the variousvgases and vapors present inthe unknown sample can be determined. However, it is usually unnecessaryto take all these factors into account in a single analysis.

According to the present invention. I take advantage of the fact thattherelative amounts of a given type of ion produced from diiferentcomponents generally varies with the ionization p0- tential. Thus, forexample, I may determine the composition of a normal butane-isobutanemixture by measuring the number of ions of a single mass-to-charge ratioderived from such a mixture at two diierent ionization voltages. Bycomparing these results with. measurements obtained from pure componentsunderlow pressure conditions such as those already hereinabovedescribed, I may compute the quantities of normalbutane and isobutanefrom equations similar to equation 1. My method may. of course, beapplied to mixtures of gases other than hydrocarbons.

In order to determine how the intensity of any V particular ion beamvaries with the energy of the electrons in the electron beam, I adoptone of two procedures. 4-In the rst, the total accelerating voltage towhich ions are subjected may be set at a predetermined value byadiustment of the total potential between pusher electrode 43 andcollimating slitelectrodes 4| and 41 by suitable positioning of theslide contact 01 on potentiometer 69 in the deflection control circuitGi, thereby focusing ions of any predetermined mass-to-charge ratiodesired at the exit slit 5l. With the mass spectrometer adjusted todetect ions of predetermined mass-tocharge ratio, the energy of theelectrons in the electron repeated for each peak o: interest in eachcomponent of interest. In the second procedure I l set contact 38successively at different points of rheostat Il to establish differentvalues of electron accelerating voltages as indicated by a voltxneterI6, and obtain a mass spectrum of the sample under investigation at eachof these voltages for each component in which I am interested, by themethod hereinabove described.

In both of these methode the voltage between the filament Il and theelectron beam intensity control electrode 2i is maintained constant andpreferably at a value such that the'total electricai current'borne byelectron beam i8 is constant.

In Fig. 3, I have plotted typical ionization curves a and b for ions ofthe same mass-tocharge ratio obtained from equal amounts of two purecomponents. The appearance potentials,

that is the potentials below which no ions are formed, differ betweenthe respective components in this case. Curve c is a plot of the ratioof the amounts oi ions formed from the two components at the respectivevoltages. Curve d is a curve obtained for such ions from a mixturecontaining 'half of said amount of each component. Though the ionizationlcurves of the individual components are relatively smooth, the curve dfor the mixture has a sharp bend at point p, givingv an indication ofthe character 'of the mixture. While the composition of the mixture maybe determined by comparison of curves a, b, and d, the analysis of themixture may be made more simply by solving a pair of simultaneousequabeam may be adjusted to successively different tions, of the typegiven in Equation 1 above. corersponding to two voltages at which thecomponents produce such ions in different proportions. For example, ifthev respective ion beam intensities ot a sample o f the mixture andsamples of the components are measured at both, 50 volts and 20 volts,and these are the only two components present in the mixture, thefollowing relations hold:

wPIM: 13.7 (4) and t and the sensitivity coemclents for the secondcomponent at the'two voltages are If in a particular instance theintensity of the beam obtained for the mixture at 50 and 20 voltsrespectively are:

Ila-@215.7 (l0) and Y the amounts of the components present in themixture will be My invention may, of course, be applied to cases inwhich more than two components are present in the mixture..` vIt 'mayeven be applied if more than two components produce ions of the samemass-to-charge ratio. In this case a number of variations'ofmy inventionwill naturally occur to those skilled 'in the art, in view of theforegoing explanations. For example, an analysis of a three-componentmixture may be made readily by measuring the intensity of a single ionbeam at three voltages. if theshapes of the ionization curves aresuiliciently different for the three components. In the event that theshapes of the ionization energy curves for two' components are the samefor a particular io`n beam to which another component having-anionization curve of different shape makes a contribution,

. additional dataare-necessary in order to perform a complete analysis.Such additional data may be 'in the form of ion beam intensity andsensitivity coeflcients for ions of another massto-charge ratio,applicable to the mixture and its components. In any case it will beclear that an analysis containingn-components may be obtained fromcomputations based on n independent measurements obtained in a massspectrometer on the mixture, provided, of course, corre-v sponding datafor the individual components are f known or determined.

From the foregoing description it is seen that I have provided anewmethod of mass spectrometry which makes possible the determination ofthe amounts of a greater number of components presentin a mixture than'there are ions of diierent'mass-to-charge ratios measured.

1. In the analysis of a mixture containing a plurality of componentsinvolving the comparison of measurements of the quantities of ions ofpredetermined mass-to-charge ratios formed in a potential.

mass spectrometer from said mixture and from reference samples includingdifferent proportions of the components under. the same ionizationconditions, the improvement which comprises separately ionizing saidmixture at dierent ionization voltages at which ions of one such massto-charge ratio are formed in diierent proportions from the respectivecomponents,and measuring the amounts of such ions formed at each saidvoltage from the vmixtureand from the several reference samples.

2. In the analysis of a mixture by mass spec- 'l trometry theimprovement which comprises bombarding the mixture with an electron beamhaving a potential such that ions of a common massto-charge ratio areformed from a plurality of components in the mixture, bombarding themixture with an electron beam-having a different potential but one suchthat ions of the common mass-to-charge ratio are formed from the samecomponents, but the proportions of the ions de'- rived from thecomponents being different than the proportions derived from thesecomponents at the other potential and measuring the amount oi? the -samemass-to-charge ratio such that the ions of the plurality of componentsare in a dif. ferent-ratio to each other than .when under theymst-mentioned bombardment and measuring the amount of the ions of saidmass-to-charge ratio formed from the mixture at each ionization REED C.LAWLOR.

